The hot-melt pressure-sensitive adhesive means a pressure-sensitive adhesive which is melted under heating. The hot-melt pressure-sensitive adhesive can be coated on an adherend with no solvent, is solidified by cooling, and exhibits pressure-sensitive adhessiveness in a solidified state. The hot-melt pressure-sensitive adhesive is used for a wide variety of pressure-sensitive adhesive products in paper processing, wood working, hygiene materials, and electronic fields.
In general, the “pressure-sensitive adhesive”, i.e., PSA means a binding material which adheres only by contact, and immediately exhibits adhesive force durable for practical use without necessity for accompanying a change in state from liquid to solid. On the other hand, a binding material that requires a change in state from liquid to solid in order to exhibit adhesive force durable for practical use is referred to as an “adhesive”. When exhibiting adhesive force, the adhesive requires a process of changing from solid to liquid such as evaporation of a solvent, cooling, and a chemical reaction, and thus requires a substantial solidification time.
Hot-melt PSAs comprising various styrene-based elastomers as base polymers are known as examples of conventional hot-melt PSA. Such a conventional composition is a hot-melt-type PSA composition comprising, as a base polymer, a styrene-butadiene-styrene block copolymer (hereinafter also referred to as “SBS”) or a styrene-isoprene-styrene block copolymer (hereinafter also referred to as “SIS”).
The hot-melt PSA comprising, as a base polymer, a styrene block copolymer is frequently applied to base materials such as paper, non-woven fabric, a label, and a film, or adherends.
JP 2001-162747 A discloses a hot-melt-type PSA composition obtained by blending a styrene-based block copolymer, a tackifier resin, and a plasticizer (the claims, paragraph [0013] etc.) The hot-melt PSA in the same literature contains a styrene resin, a petroleum-based resin and/or a terpene-based resin as the tackifier resin (paragraph [0023], Table 1). It is described as follows: The hot-melt adhesive exhibits excellent adhesive force in a wide temperature range from a low temperature to a high temperature, and achieves effects such as having no contamination and stickiness due to bleeding of a plasticizer ([0018]).
However, this hot-melt PSA has a strong odor, a high load imposed on a worker and a user using PSA products, and a high environmental load. Also, its viscosity is reduced with time particularly under a high temperature environment and thus it is poor in stability with time. Further, its adhesive force decreases under a high temperature environment or a low temperature environment and thus adhesive force retention performance from a low temperature range to a high temperature range is yet to be sufficient.
JP 2004-051688 A discloses a bonding resin composition comprising a polystyrene-based resin having a high-molecular weight, crystalline syndiotactic structure as a polystyrene resin, and a styrene-based elastomer (claim 1, paragraph [0049]). However, the bonding resin composition of JP 2004-051688 A has no pressure-sensitive adhessiveness and thus is not preferred for producing PSA products. Since the bonding resin composition in the same literature has an extremely high viscosity compared with hot-melt PSAs, coating by means of a roll coater or the like is difficult.
Recently, from a rise in awareness of environmental problems, hot-melt PSAs using a non-petroleum-based resin are studied as the hot-melt PSA. However, the non-petroleum-based resin has insufficient compatibility with other components such as a tackifier resin, pressure-sensitive adhessiveness of the hot-melt PSA decreases with time.